In recent years, in the reading of ID information that is recorded on tags, systems that perform reading by utilizing magnetic coupling with tags or RFID systems that use wireless frequencies to read tag information have been developed and operated.
For the latter RFID system that performs reading by using wireless frequencies, European standards have been disclosed by the ETSI (European Telecommunications Standards Institute) (Draft ETSI EN 302 208-1V1.1.1 (2003-12)) as an example. The usage of an RFID system that employs the 865 MHz to 868 MHz band at a maximum power of 2 W was disclosed in Draft ETSI EN 302 208-1V1.1.1 (2003-12).
Here, there are cases where a plurality of reader/writers are run adjacent to one another as an RFID system. FIG. 1 illustrates an example of this state.
In FIG. 1, a signal transmitted by the respective reader/writer #1 (#2) has an interference 3 with respect to the other reader/writer #2 (#1), and there is the problem that the reception performance of the reader/writers #1 and #2 deteriorates. In order to deal with this problem, carrier sensing is carried out before transmission in order to reduce the effect of interference 3 and a search for a frequency channel that is not being used by the other reader/writer #2 (#1) is performed. Further, a method that uses a spare channel if one exists and waits until a spare channel is found if none exists has been adopted.
In other words, FIG. 2 illustrates a method using such carrier sensing. FIG. 2 is a reading operation flowchart for tag information of individual reader/writers in FIG. 1.
According to FIG. 2, each of the reader/writers #1 and #2 is constituted by a reader/writer main body section 1a and an antenna 1b. 
First, an initial channel (frequency bandwidth) is set by the reader/writer main body section 1a (step S1) and the level of the carrier received by the antenna 1b in the initial channel thus set is measured as an interference signal (step S2). If the level of the interference signal is equal to or more than a threshold value (step S3, N), this channel cannot be used and, therefore, the interference level of another channel is the measurement target. In other words, it is judged whether measurement for all of the channels has already been performed and, if channels for which measurement has not been performed exist (step S4, N), the channels are switched in sequence (step S5) in order to return to the same interference level measurement (step S2).
In cases where measurement of all of the channels has already been carried out (step S4, Y), the wait time is randomly set (step S6), the processing returns once again to the initial channel setting (step S1) after the set wait time has elapsed, and the processing is repeated.
If the interference level is equal to or less than a threshold value (step S3, Y) in the interference level measurement (step S2), a channel for which the interference level is equal to or less than the threshold value is determined (step S7) and a command signal is transmitted (step S8).
FIG. 3 illustrates a command signal emitted by the antenna 1b of the reader/writer #1 (#2) and a response signal from tag 2a (2b). As shown in FIG. 3A, the reader/writers #1 and #2 transmit an unmodulated continuous wave CW and modulate this unmodulated continuous wave CW by means of a command signal so that the continuous wave CW is transmitted using fixed right-handed polarization, for example, by antenna 1b. 
Upon receipt of command signals from the corresponding reader/writers #1 and #2, tags 2a and 2b respond by modulating unmodulated continuous carrier wave CW by taking characteristic information that contains information identifying the good (ID) to which the tag belongs as tag information (see step S9: FIG. 3B). Reader/writers #1 and #2 receive this response signal and end the communications (step S10).
In addition, another method that is used is a method according to which a control device centrally controls a plurality of reader/writers #1 and #2 and controls the transmission timing of the respective reader/writers.
A former method that proceeds according to the flow of FIG. 2, i.e. which involves performing carrier sensing as described hereinabove to search for a frequency channel that is not being used by another reader/writer and using a spare channel if such a channel exists has been confronted by the problem that there is a large number of reader/writers and the wait time is long. In addition, in the case of the latter method that involves performing centralized control by means of a control device, when a large number of reader/writers exist, there is the problem that the allocated communication time is short.